This invention relates to a bicycle frame having composite material tubes, more particularly to a joint for a bicycle frame.
Referring to FIG. 1, a common metal bicycle frame essentially includes a head tube 10, a seat tube 11, a top tube 12, a downtube 13, a seat stay 14 and a chain stay 15. Fittings 101, 102, 103, 104 and 105 are welded between the tubes 10, 11, 12, 13, 14, 15 and 16 of the bicycle frame. For example, the fitting 101 is disposed between the head tube 10 and the top tube 12 and between the head tube 10 and the downtube 13. It is understood that it is difficult to perform the welding process of these joints 101, 102, 103, 104 and 105. Furthermore, the joints are made unsightly as a result of the welding process. To overcome these disadvantages, this common bicycle frame was replaced with a unitary bicycle frame of a composite material, such as carbon fiber. To strengthen bicycle frames of a composite material, the curved portion between any pair of tubes is laminated. As a result of the laminating process, the cross-section area of the curved portion is increased making the manufacturing process inefficient and thereby raising manufacturing costs. Accordingly, it is desirable that suitable fittings are developed to interconnect available composite material tubes, e.g. a top tube, a downtube and a seat tube for a bicycle frame, so that the lightweight advantage of unitary composite material bicycle frames is still remained. U.S. Pat. Nos. 932,546 and 4,721,407 disclose a joint which includes a fitting with a tube sleeved and adhered thereon. Although the fittings of these patents may be used to interconnect available composite material tubes so that no welding is needed, they have the following disadvantages: (1) If they are made of a composite material, an unexpected laminating process is required; and (2) If they are made of a metal, the outer appearance of the bicycle frame is unsightly.